A variety of factors affect the top speed of a watercraft that is jet pump driven. Discharge nozzle size is one such factor. Reducing the size of the jet pump discharge nozzle increases the velocity of water exiting the pump, assuming a constant volumetric flow rate of water. Accordingly, at the maximum volumetric flow rate of a jet pump, reducing discharge nozzle size increases top speed. Unfortunately, reducing discharge nozzle size also restricts water flow through the nozzle and reduces volumetric flow rate in the lower range of pump operation. Lower volumetric flow rate thus impairs acceleration at speeds below the top speed of a watercraft and may be particularly noticeable when accelerating from a standstill.
Conversely, increasing discharge nozzle size reduces top speed since, at the maximum volumetric flow rate of the watercraft, discharge velocity is reduced. Even so, enlarging discharge nozzle size bears the advantage of improving acceleration at lower speeds. Conventionally, watercraft manufacturers selected a balance between top speed and acceleration and established discharge nozzle size accordingly. Some individuals desiring a different balance between top speed and acceleration chose to bore the discharge nozzle of their watercraft, increasing discharge nozzle size. Others used nozzles having adjustable sizes, for example, by installing different size nozzle rings.
Eventually, manufacturers began to use the tail cone of a jet pump as one of its tuning components. The tail cone typically mounts to the pump stator and functions as a cap for the impeller shaft bearings. By virtue of its cone shape, a tail cone may also assist with hydrodynamic flow through the jet pump, refocusing water flow into a solid jet stream as it exits the discharge nozzle. For example, the size and position of the tail cone may be altered to vary back pressure in the discharge nozzle area. As known to those of ordinary skill, increasing back pressure can yield more efficient processing as water moves through the pump so as to increase volumetric flow rate and, accordingly, acceleration. However, such efforts attempted in the past did not produce improvements in top speed. Other tuning efforts have included a lengthened tail cone with an attached rod extending from the tail cone into the discharge nozzle. The rod reduces the overall area of the discharge nozzle cross section and the effect is identical to decreasing nozzle size. Namely, top speed may increase, but a jet pump fitted with such a tail cone/rod assembly may provide less acceleration at lower speeds.
As may be appreciated from the discussion above, conventional jet pump performance improvements have included either improving acceleration or improving top speed, but not both. Additionally, improvements gained in acceleration sacrificed top speed and vice versa. The best balance between acceleration and top speed often may be the original nozzle installed by the manufacturer. Accordingly, jet pump driven watercraft exhibit a very limited range of tuneability. At least for the reasons indicated, a desire exist to enhance jet pump top speed without sacrificing acceleration and perhaps to even improve acceleration at lower speeds while still increasing top speed.